1. Field of the Invention
The present invention relates to an electric damper for a vehicle suspension device.
2. Description of the Related Art
JP 2007-276571 A discloses a technology relating to an electric damper (motor-driven damper device) for a vehicle suspension device having a rack and pinion mechanism. Moreover, JP 2005-256921A discloses a technology of detecting a vertical acceleration parts on the spring of a vehicle using an acceleration sensor provided right above each wheel to detect the vertical acceleration, acquiring the power spectrum distribution of the vertical acceleration, reading out a power spectrum in a frequency region relating to goodness/poorness of the ride comfort, and changing the cutoff frequency of a low-pass filter in accordance with the sum of the spectra, thereby controlling the damper characteristic.
According to JP 2005-256921A, however, if the electric damper is set to have a characteristic with a high damping force, i.e., a characteristic for a sporty vehicle that a ride comfort is hard, wheel vibrations (hereinafter, “unbalance wheel vibration”) caused due to the unbalance of a wheel are transmitted to a vehicle body on the spring, so that a driver feels uncomfortable vibrations.
It is possible to reduce transmission of the unbalance wheel vibration to the vehicle body by setting the damping force in a high frequency region low. However, setting the damping force to vertical vibration of a wheel low is not desirable since vehicles having a ride comfort set to a hard characteristic are generally sporty vehicles, and the ride comfort is set hard to drive the vehicle with the driving characteristic being emphasized while acquiring road-surface information from vibrations from a road surface. Because the unbalance wheel vibration is not good for transmitting road-surface information, but the unbalance wheel vibration obscures road-surface information, so that the unbalance wheel vibration is not desirable for sporty vehicles.
The present invention is made to overcome the foregoing problem, and it is an object to provide an electric damper that does not transmit unbalance wheel vibration to a vehicle body to realize comfort driving.